Ok I'm glad we're getting somewhere.
Fuel prices are different everywhere, I'll stick £10 in before I go home tonight and then do a full tank then (£80). Also as this is a website about fuel economies you can increase your economy by not driving around with a full tank. I think this is a fairly key point for this site.
Anyway Cider your new example isn't right either, sorry
"Tank was full after the 50L fill, so there is 100L in it.
Some driving done, 10L put back in, not full. For arguments sake, lets say it has 99L in it. That would be a best case scenario. 10L bought, 11L used
Some more driving done, 20L put back into it. Again, let us assume that it is back up to 99L. 30L bought, 32L used.
Buying 30L can not get it back up to 100L. That is my point."
No, but you do not have a full tank. So do a FULL tank fill up at the end and tally it all up
You don't specify how much you used in the second bit of driving, but as you're now up to 99ltr having put 20ltr back in, it must have been down to 79ltr. So you used 20ltr. So 30ltr of fill ups, you've used 31ltr, you don't know how much is in the tank but you said it was 99ltr. So top it up to a FULL tank (our reference) = 1 extra litre. So 31ltrs.
Basically there is no magic, whatever you put in is either in the tank still or has been used. I suppose simply put
If I have a full tank, drive 100 miles, and put 50 litres of fuel in to get back to full
is that any different to
I have a full tank, drive 50 miles, put in 25 litres, drive fifty miles, put 25 litres of fuel in
Assuming both examples return the same economy either way I've driven 100 miles and consumed 50ltrs. One is one full fill up, one is one partial fill up followed by a full fill up.
Anyway sorry to write so much and it gets very wordy, back to my original point I'd love partial fill ups to count as most of the time people here don't do full fill ups. If we did it once in a while to get an average that'd would suit us well